WASHINGTON, March 27 (Reuters) - California is prepared to sue the Environmental Protection Agency if it tries to weaken Obama-era vehicle efficiency standards, the state’s attorney general said on Tuesday, setting up another in a series of legal fights between Sacramento and the Trump administration.

The EPA is expected to declare by April 1 that current rules aimed at doubling average fleet-wide fuel efficiency to about 50 miles (80 km) per gallon by 2025 are “not appropriate,” sources told Reuters last week.

The declaration would enable the EPA to take steps to weaken those federal standards, which had been implemented by the administration of former President Barack Obama, a Democrat, with the backing of automakers.

“We are going to do everything that can been done to defend these standards,” California Attorney General Xavier Becerra told Reuters in an interview. “So far, when we have been challenged on environmental standards we have had a good record in court. We haven’t lost a case.”

In 2011, California’s air emissions regulator and the Obama administration reached an agreement with major automakers to nearly double average fleetwide fuel efficiency by 2025, but included a “midterm review” to determine by April 2018 whether the final requirements were feasible.

But if the administration of Republican President Donald Trump weakens federal auto efficiency standards, then California - the most populous U.S. state and a massive market for automakers - could enforce its own stricter rules that nearly a dozen other states, including New York, would follow.

Carmakers are worried that such a dispute would lead to a patchwork of different standards across the country.

EPA Administrator Scott Pruitt told Reuters in January that “California doesn’t have the authority to set the standards for the rest of the country” - leading some officials in the state to worry the administration could attempt to revoke Sacramento’s authority to set its own rules.

EPA spokeswoman Liz Bowman confirmed its recommendation is under interagency review and will be completed by the April 1 deadline.

Becerra said he is confident courts would side with California if EPA tries to weaken the federal standards or, further down the road, undercut the state’s special authority under the federal Clean Air Act to set its own.

“We are not yet at round one and we think we have some good knockout punches for the next rounds,” he said. (Additional reporting by David Shepardson in New York, editing by G Crosse)